Shock Absorbers

I have a 2002 TB LTZ with 34K. What is the life expectancy of the shocks, and what should they be replaced with?

I have no idea around what mileage to replace them at, my 02 with 48k still has the original shocks. I hear the heavy duty bilsteins are good, and a little stiffer than the stock ones. BTW, the stocks are Bilsteins as well. Erik